1. Technical Field
The present invention relates generally to systems for drying objects and materials. In particular, the present invention relates to such systems that employ heat exchangers incorporating refrigeration cycles such that only minimal heat and water vapor are exhausted to the ambient atmosphere. More particularly, the present invention is directed at providing such systems that can operate at drying temperatures considerably higher than has been possible heretofore for dehumidification dryers. More particularly yet, the present invention is directed at providing such systems in which the objects and materials to be dried can be maintained during the drying process at temperatures at least as high as 250 degrees Fahrenheit.
2. Description of Prior Art
Although all drying involves “dehumidification” of the object to be dried, the term is used in the industry to refer to systems that heat the objects to be dried by circulating a hot, relatively dry atmosphere past and through them, and then conveying that atmosphere into a dewatering region for drying before re-introducing it to the objects to be dried. In this way, the drying atmosphere arrives in the drying region with relatively low humidity and leaves it containing water vapor evaporated from the objects to be dried. Dehumidification dryers are generally closed systems, in contrast to drying systems where the objects to be dried are simply heated to a high temperature and the resulting gaseous water (a “greenhouse gas”) are vented into the ambient atmosphere along with volatile organic compounds (VOCs) and other pollutants. Also, being closed systems, dehumidification dryers do not discard the large quantities of heat (energy) that are vented by traditional systems, and hence consume considerably less fuel.
In the field of industrial drying, the use of refrigeration apparatus as an integral part of dehumidification equipment is well known. The dewatering process typically draws the warm, humid air departing the drying region across a refrigeration coil through which liquid refrigerant is circulated. Heat is conveyed from the warm moist air past the coil, where this heat is transferred to the refrigerant, serving as the heat of vaporization that converts the liquid refrigerant into a gas. For this reason, the coil is referred to as the “evaporator” portion of the refrigeration circuit, or simply, the evaporator.
Overall, the refrigeration circuit includes the evaporator followed by a compressor, where the now-gaseous refrigerant is compressed, and a condenser, where the refrigerant's heat of vaporization is shed and the refrigerant is reconverted to a liquid. In order for the drying atmosphere to be dewatered, its temperature must be cooled at the evaporator to a temperature below the dew point of the moisture laden air. Once it has passed that point, it is reheated before being returned to the object to be dried, the reheating being done in whole or in part by the heat coming off the refrigeration circuit's compressor.
Despite the efficiencies and other desirable features offered by dehumidification dryers, they have had relatively limited use in large-scale drying, specifically in the lumber industry. This is because of the limitations on the operating temperatures hitherto attainable by dehumidification dryers. Certain species of wood, such as softwoods, require drying temperatures in excess of 250° F. for commercially efficient drying (lower temperatures may be used, but the drying time is far longer and can lead to degradation of the product, and thus are undesirable). For temperatures demanded for these commercial drying operations the available refrigerants break down chemically or become ineffective for other reasons, including the high pressures they rise to upon receiving drying atmospheres at these high temperatures, to the point where the resulting load placed on the compressor motor causes that motor to fail. For these reasons, straight dehumidification dryers were limited to a maximum drying temperature of about 180 degrees Fahrenheit, whereby they were precluded from use in a large number of drying operations.
The phrase “straight dehumidification” refers to a system whereby all of the atmosphere (air) leaving the drying region is passed over the evaporator for dewatering. If the temperature of that air upon arriving at the evaporator exceeds about 120° F., the heat that must be transferred to the refrigerant in order to lower the air's temperature to its dew point causes a breakdown of the refrigeration sequence, for the reasons just stated. This problem was partially alleviated by the modifications taught by Lewis, U.S. Pat. Re. 31,633 (1984), (hereinafter, “Lewis (1984)”), which coupled a feedback mechanism to an air-diverting scheme, whereby the volume of air being introduced to the refrigeration unit per unit time is varied as a function of the leaving air or refrigerant temperature. By putting a cap on the amount of heat being dumped into the refrigerant, the drying atmosphere (and hence the objects to be dried) could be raised to higher temperatures, i.e., as high as 160 degrees Fahrenheit. However, Lewis (1984) failed to separate the dehumidifying unit from the drying chamber, thereby preventing the use of higher temperature refrigerants. This problem was then alleviated to some extent by the additional modifications taught by Lewis, U.S. patent application Ser. No. 10/402,007 (2003) (hereinafter, “Lewis (2003)”), which taught the use of an improved refrigerant which could operate at temperatures of up to 225 degrees Fahrenheit in conjunction with locating the dehumidifying unit outside the drying chamber. This was still too low for certain drying needs, though.
A feature of both the Lewis (1984) and the Lewis (2003) drying apparatuses is that the air-intake to the dewatering region includes a mechanism for variably diverting a fraction of the air coming from the drying region, so that that fraction does not come into contact with the coil. The goal is to keep the temperature of the refrigerant or air leaving the coil below a pre-defined level. This is done by coupling the diversion mechanism to a sensor monitoring the temperature directly, or monitoring some surrogate for it. When the monitored temperature exceeds its preset maximum, an increased fraction of the humid air coming from the drying region is diverted around the coil, thus reducing the heat load that the coil has to handle. The systems of Lewis (1884) and Lewis (2003) therefore permit higher drying temperatures to be used while retaining the advantages of the closed-system dehumidifier. In addition to permitting higher drying temperatures, it allows a much more efficient use of “cooling” energy toward the end of the drying regime, when the humid air is far less humid that at the outset of the regime. During that stage in the drying, the difference between the air temperature and the dew point may be quite large with the result that in order to condense water out of that air, it is necessary to lower the temperature of the air many degrees. In this case, even if the temperature of the air exiting the drying region does not exceed the maximum operating temperature of the refrigerant, straight dehumidification schemes may not work, simply because the circuit is unable to remove enough heat to lower the temperature of the complete flux of the drying atmosphere below that atmosphere's dew point. If the air flows past the evaporator without being lowered in temperature below its dew point, it emerges with the same absolute humidity that it had upon entry and consequently will serve no further drying function upon being reheated and directed across the object to be dried. Under these circumstances, the diversion systems of Lewis (1984) and Lewis (2003) again provide an advantage. By permitting just a small fraction of the total drying-atmosphere flux to contact the evaporator coil, that fraction can be reduced to below its dew point and hence dewatered. This will result in an overall reduction of humidity of the entire flux of the air once it has been reunited for its next pass across the objects to be dried. This not only allows the conventional drying schedules for some woods to be met with a dehumidification drying system, but allows all substances to be dried in dramatically shortened times, and without the costs in energy and pollution that are associated with open systems. The system of Lewis (1984) permits drying temperatures as high as 160 degrees Fahrenheit to be reached while using conventional refrigerants at pressures used in conventional refrigeration equipment, while the improved system of Lewis (2003) permits drying temperatures as high as 225 degrees Fahrenheit to be reached while using high-temperature refrigerants at pressures used in conventional refrigeration equipment.
Even though dehumidification dryers at drying temperatures as high as 225 degrees Fahrenheit represent a great improvement, there still remain certain woods, such as Southern Yellow Pine (the common name for the species Pinus taeda, Pinus palustris, Pinus echinata, and Pinus elliottii), that require even higher temperatures at least in some portion of their normal drying schedules. For example, Southern Yellow Pine is best dried at temperatures of between 240 degrees Fahrenheit 260 degrees Fahrenheit. Even for materials that do not require the higher temperatures, the drying speed is normally increased by using higher temperatures. That is, whenever the drying temperature is increased, the rate of drying available for all objects to be dried goes up dramatically, regardless of whether they require the high temperatures to permit them to be dried in accord with a conventional drying schedule. Given the exigencies related to minimizing all kinds of pollution and maintaining energy efficiency, any improvement in drying systems must involve closed systems or systems considerably more closed that the conventional ones, regardless of its detailed structure and operation. Although closed-system commercial wood dryers incorporating a refrigeration circuit do exist, they have not been able to be operated at the higher temperatures available to open wood drying systems.
Therefore, what is needed is a closed drying system that permits drying temperatures significantly above 225 degrees Fahrenheit to be maintained. What is also needed is such a closed drying system that can be incorporated relatively easily into existing closed-system drying apparatus, and in particular to dehumidification dryers.
It is therefore an object of the present invention to provide a closed drying system that permits drying temperatures significantly above 225 degrees Fahrenheit to be maintained.
Another object of the present invention is to provide a closed drying system that can be incorporated relatively easily into existing drying apparatuses.
Other objects and advantages of the present invention will become obvious to the reader and it is intended that these objects and advantages are within the scope of the present invention. To the accomplishment of the above and related objects, this invention may be embodied in the form illustrated in the accompanying drawings. Attention is called to the fact, however, that the drawings are illustrative only, and that changes may be made in the specific construction illustrated and described within the scope of this disclosure.